The present invention relates to a permanent magnet type rotary machine employed in office automation machines, e.g., copier, printer, peripheral equipments of a computer system, vehicles, factory automation machines, e.g., conveyor, etc.
There are many permanent magnet type rotary machines—such as a PM type step motor having a permanent magnet rotor, a hybrid (HB) type step motor having a gear-shaped iron core and a permanent magnet, and a brushless motor having a magnetic pole detecting unit for feedback control. A permanent magnet type rotary machine, which has comb-shaped magnetic pole teeth facing a rotor, includes: a plurality of stator units stacked in the axial direction; and a rotor having a permanent magnet, which faces the stator units and is alternately magnetized as N-poles and S-poles. By passing a current through coils of the stator units with switching the current direction, stator magnetic poles and rotor magnetic poles mutually attract and repel so that the rotor can be rotated.
In the permanent magnet type rotary machine having the comb-shaped magnetic pole teeth facing the rotor, number of the stacked stator units is equal to number of phases of the magnetizing current. For example, in case of a two-phase motor, an A-phase stator unit and a B-phase stator unit are stacked. Torque of a step motor is resultant torque of torque generated by a magnetizing current and cogging torque (detent torque) generated by variation of magnetic resistance between a rotor and stator yokes. Cogging torque elements become high harmonic elements of motor torque, so rotational fluctuation and vibration are caused. Thus, the same step motor is serially connected to an output shaft of said step motor, which is an example of the permanent magnet type rotary machine, and their phase are mutually shifted a half of a detent torque cycle in the rotational direction, so that vibration elements caused by the detent torque can be reduced (see Japanese Patent Gazette No. 9-163798).
Further, to rapidly reduce motor vibration caused by high harmonic elements included in waves of an induced back electromotive force, a step motor, in which areas of comb-shaped teeth of stator yokes and clearances therebetween are varied so as to offset third high harmonic waves and reduce vibration, has been developed (see Japanese Patent Gazette No. 10-127024).
These days, high torque small motors have been required. Thus, a motor having a rotor, whose magnet is made of a rare-earth metal, e.g., Nd—Fe—B, with high maximum energy product, has been developed. By using such rare-earth metal, a high torque small motor can be realized.
Large torque can be generated, however, amounts of magnetic flux significantly vary in magnetic circuits formed between stator yokes and a permanent magnet. Therefore, cogging torque and rotational vibration are increased, and rotational fluctuation become worse.
In case of using the rare-earth magnet whose maximum energy product is high, the amounts of magnetic flux passing the magnetic circuits are increased, so the magnetic flux passing the magnetic circuits of the stator yokes are easily saturated. To solve this problem, thicknesses of the stator yokes must be thicker. However, in the permanent magnet type rotary machine having comb-shaped magnetic pole teeth, it is difficult to precisely form the stator yokes by press working. Further, a die set for the pressing work must be large, so that a huge manufacturing facility must be required and the production cost must be increased.
On the other hand, a plurality of stator units may be stacked. In this case, thicknesses of stator cores are not changed, but stator yokes are piled each other. However, magnetic circuits are formed between a stator and a permanent magnet, depending on a rotational position of a rotor, and cogging torque is newly generated by variation of amounts of magnetic flux in magnetic circuits formed between magnetic pole teeth of the adjacent stator yokes, so that the resultant cogging torque of a motor must be increased.